Additive blending
A blend mode that adds the source colour on top of what is already in the buffer — light piles onto light.
The formula is dst = src + dst, clamped at the top. Because addition is commutative, draw order does not matter, and every layer only brightens the image. That makes it ideal for fire, sparks, laser beams, magic and any glow where overlapping particles should get brighter rather than occlude one another.
The contrast is alpha blending, which weighs by transparency and is order-sensitive. In OpenGL additive blending is simply glBlendFunc(GL_ONE, GL_ONE) — two lines of code behind half the particle effects on screen.